Hey, i got my final GCSE maths test (non calc) which starts in around 1 and a half hours!

I don't get this question:

EDIT: HERE IS THE GRAPH. SORRY IN A RUSH AND FORGOT LOL.

A, B, C and D are points on the circumference of a circle, centre P.

AC is the diameter of the circle.

Angle DAC = 20°

(a) Find the size of angle ACD

(b) Find the size of angle DBC

Give a reason for your answer